The invention relates to a circuit for the line deflection in a picture display device, comprising the series arrangement of a controllable semiconductor element, a controllable bidirectional deflection switch and a supply inductance, which series arrangement is connected between the terminals of a supply voltage source, further comprising a deflection resonant network including a line deflection coil which network is arranged parallel to the deflection switch, a first drive stage for applying a first drive signal to the semiconductor element for controlling the energy stored in the inductance with a feedback control in dependence upon the amplitude of the generated line deflection current flowing through the line deflection coil for maintaining the said amplitude constant, and with a forward control in dependence upon a field frequency signal for obtaining a field frequency variation of the amplitude of the line deflection current, and a second drive stage for applying a second drive signal to the deflection switch for line frequency rendering this switch alternately conducting, namely during the trace time of the line deflection current, and non-conducting, namely during the retrace time of the said current.
A circuit of this type has been proposed in the non-prepublished European Patent Application 195,392. A substantially constant value can be obtained for the amplitude of the line deflection current and hence for the width of the picture displayed, in spite of variations of, for example, the supply voltage and/or of loads connected to the circuit. This is ensured by the feedback control with which as information regarding the said amplitude, a direct voltage obtained by rectification of a pulse present in the circuit is applied to a comparison stage. In this stage a comparison is effected with a reference and the output signal of the comparison stage is applied to the semiconductor element for controlling the voltage across this element.
The raster distortion of the displayed picture is corrected by means of the forward control, and the variation caused thereby is superposed in a simple manner on the variation caused by the feedback control. The forward control is active throughout the field period, whereby the desired field-frequency variation is introduced accurately and without inertia. However, the variation of the line amplitude introduced in this way is counteracted by the feedback control and in order to still obtain an adequate variation the feedback control has to be slow with respect to the desired field frequency variation, which can be inadmissibly slow in view of the low value of the field frequency, so that this control is not capable of quickly reacting to changes in amplitude.